Interference suppression, method and device, in reception, for a wideband radio signal

ABSTRACT

Apparatus and methods degarble a received broadband radio signal using spatial filtering. The apparatus and methods use various garbled versions of the signal delivered by parallel processing chains assumed to have the same transfer function. Automatic equalization ( 4   d ), at the level of the outputs of the processing chains ( 2   a  to  2   g ), is performed before the spatial filtering ( 3 ). The automatic equalization reduces disparities, innate or originate from drifts over time, between processing chains ( 2   a  to  2   g ), and uses a local test signal generator ( 5 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on International Application No.PCT/FRO2/03001, filed on Sep. 3, 2002, which in turn corresponds toFrench Application No. 01 11616, filed on Sep. 7, 2001, and priority ishereby claimed under 35 USC §119 based on these applications. Each ofthese applications are hereby incorporated by reference in theirentirety into the present application.

FIELD OF THE INVENTION

The present invention relates to the antigarbling, in reception, of abroadband radio signal by a spatial filtering taking account of variousdeformed versions of the signal that are delivered by parallelprocessing chains, such as for example, the individual processing chainsassociated with the radiating elements of a receive array antenna.

BACKGROUND OF THE INVENTION

Certain radio systems such as, for example, the so-called “GPS”(standing for “Global Positioning System”) satellite navigation systemuse spread spectrum, hence broadband, radio signals received with a verylow power level, easily, from 30 dB to 40 dB below the level of thermalnoise at the input of a receiver. In a favourable environment, that isto say one which is not very noisy, the despreading operation enablesthe signal-to-noise ratio of the signal received to be raised well above0 dB.Hz, so that the signals received may be utilized safely.

SUMMARY OF THE INVENTION

The technique of spread band transmission is relatively insensitive tointentional jamming when the frequency band used is not known to oneseeking to jam since this frequency band is difficult to tag in view ofthe low power density per hertz of band of the signal transmitted. Onthe other hand, this is no longer the case when the frequency band orbands used are prescribed. It is in fact easy to disturb the receptionof a spread band transmission signal by drowning it in broadbandjamming. Specifically, by transmitting throughout the band occupied bythe useful signal, noise exhibiting a power density per hertz that isappreciably greater than that of the useful signal it is possible todegrade the signal-to-noise ratio at the input of the receiver and thisdegradation propagates along the signal processing chain. The latterthen only has to reduce the signal-to-noise level below a few dB.Hzafter despreading in order for the useful signal to become difficult toutilize.

In the case of GPS, the frequency bands used are well known and thepower of the signals received is low originating as they do fromtransmitters of a few tens of watts stored on board satellites orbitingthe terrestrial globe at a distance of some twenty thousand kilometresfrom its surface. These two characteristics of prescribed transmissionfrequency bands and of low transmission powers render the reception ofGPS signals particularly sensitive to broadband jamming since, in orderto disturb the reception of GPS signals, one only has to locally employa transmitter of noise occupying the GPS frequency bands, in therelevant zone, either on the ground, or above the ground onboard anaircraft: airplane, missile or the like. Hence, GPS jamming equipmentcapable, with a relatively low power being as it is of the order of fourwatts, of disturbing the reception of GPS signals in a very extendedzone of up to 200 km in radius has appeared on the market since 1997.

To render GPS receivers less vulnerable to broadband jamming, variousmeasures have been proposed. The main ones exploit the fact that thetransmitters of useful signals, which navigation satellites are, and thejammers do not occupy the same geographical locations. They act on theradiation pattern of the antenna of the receiver to favour reception inthe directions from which the useful signals transmitted by thenavigation satellites originate to the detriment of reception in thedirections of the jamming signals.

A first known measure consists in using in reception, an antenna with askyward directed hemispherical radiation pattern that excludes thedirections with a low angle of elevation. Such an antenna is often ofarray type, with several radiating elements placed, on an earth plane,at the vertices of a regular polygon and possibly a central radiatingelement, and with a spatial combiner that produces a weighted sum, inamplitude and in phase, of the signals picked up individually by thevarious radiating elements of the antenna so as to obtain a receptionpathway corresponding to the desired radiation pattern.

A second measure complementary to the first, consists, since one is inthe presence of an array type receive antenna, in implementing thetechnique of dynamic reduction of the power of the garbling signals bycancellation of the side lobes, a technique known by the abbreviationCSLC standing for “Coherent Side Lobe Cancellation”.

This CSLC antigarbling technique was originally developed within theradar sphere. It consists in forming, dynamically, from the signalspicked up by several radiating elements, a reception pathwaycorresponding to a radiation pattern exhibiting one or more lobes in thedirections of the useful signals and holes in the directions of thejammers. Its implementation involves the following main steps:

-   -   static formation of several independent reception pathways on        the basis of the signals picked up by the radiating elements        used for reception, the independence between reception pathways        signifying that none of them reduces to a simple linear        combination of the others,    -   election from among the reception pathways formed, of a main        reception pathway, the others being regarded as auxiliary, and    -   dynamic formation of a so-called “degarbled” reception pathway        by adding, to the signal of the main reception pathway, a linear        combination of the signals of the auxiliary reception pathways        weighted in amplitude and in phase with the aid of dynamically        adjusted coefficients so that the signals of the so-called        “degarbled” reception pathway and of the auxiliary reception        pathways are decorrelated.

It is shown that the possible number of independent reception pathwaysis less than or equal to that of the distinct radiating elements of thereceiver antenna and that, in order to be able to eliminate N jammers,there must be at least N independent auxiliary reception pathways andhence an array antenna comprising at least N+1 distinct radiatingelements must be available for reception.

The efficiency of this CSLC antigarbling technique depends on theprecision and accuracy of determination of the amplitude- andphase-weighting coefficients used to add the signals of the auxiliaryreception pathways to the signal of the main reception pathway. Withoutparticular precautions, the use of this CSLC antigarbling techniquewithin the sphere of receivers of signals transmitted in spread bandmode such as GPS receivers, makes it possible to decrease, by somethirty dB, their sensitivity to broadband jamming.

One of the reasons advanced for the lack of accuracy of mechanisms fordetermining the amplitude- and phase-weighting coefficients used to add,to the signal of the main reception pathway, the signals of theauxiliary reception pathways within the framework of the CSLCantigarbling technique is the disturbing of these determining mechanismsby narrowband jamming that does not necessarily stem from hostileintention but from the ever greater congestion of the radio spectrum andgets superimposed on broadband jamming.

It has been proposed to remedy same by employing, at the level of eachof the main and auxiliary reception pathways, before the dynamicformation of the “degarbled” reception pathway, narrowband rejectionfilters whose centre frequencies are adjusted adaptively to those ofnarrowband jamming signals which are manifested by spikes that overshootthe noise level in the useful band.

In fact, other possible reasons exist, including the fact that themechanisms for determining the amplitude- and phase-weightingcoefficients employ methods of determination based implicitly onidentity of the transfer functions of the processing chains encounteredin the various reception pathways whereas the analogue nature of theinput stages of the processing chains enables this identity of transferfunctions to be approximated only imperfectly, with discrepanciesvarying disparately over time.

Specifically, the transmission frequency span of most radio signals,this being the case for signals from GPS positioning satellites, is toohigh to lend itself directly to digital processing. The radio signalpicked up must then undergo, in head-end analogue stages, rejection ofthe noise out of band and a drop in frequency, to a lower frequencyspan, intermediate frequency band or baseband, more accessible to thecurrent technology of analogue/digital converters. This analogue natureof the input stages of the processing chains and the extended frequencyrange to be passed for the processing of a spread band signal make itimpossible, even starting from an identical design, to succeed in givingtwo distinct processing chains exactly the same transfer function. Thus,undesired discrepancies of amplitude and of phase dependent on thefrequency considered, which, moreover, change in disparate and randomways over time, always exist between the transfer functions of thevarious chains. These inevitable changing discrepancies unfavourablyinfluence the precision and accuracy of the determination of theamplitude- and phase-weighting coefficients and therefore the efficiencyof the degarbling.

An aim of the present invention is to improve the efficiency of the CSLCantigarbling technique within the framework of useful broadband signals.Its aim is, more generally, to improve the efficiency of all thebroadband signal processing taking account of various versions of oneand the same signal originating from parallel processing chains assumedto have identical transfer functions.

Its subject is a process of antigarbling, in reception, of a usefulradio signal, by a spatial filtering taking into account various garbledversions of the signal that are delivered by a set of parallelprocessing chains assumed to have identical transfer functions, which isnoteworthy in that it consists in carrying out an equalization, at thelevels of the processing chains, before the spatial filtering, byindividually equipping processing chains with adjustable temporalequalization filters and with equalization filter tailoring circuitsoperating on the basis of the differences noted between two versions ofone and the same test signal produced locally, in reception, by a testsignal generator, one of the versions traversing the equalizedprocessing chain and the other serving as reference.

Advantageously, the test signal is distinguished from the useful signalby its appreciably greater power which relays the useful signal at thelevel of environmental noise.

Advantageously, the test signal is distinguished from the useful signalby an auxiliary modulation.

Advantageously, the test signal is distinguished from the useful signalby an auxiliary modulation by a pseudorandom binary string, the testsignal being added to the useful signal in its modulated form during itspassage through a processing chain and then separated from the usefulsignal by demodulation before being used by an equalization filtertailoring circuit.

Advantageously, an equalization filter tailoring circuit uses theversion of the test signal that has traversed the processing chain to beequalized to measure, explicitly, the transfer function of thisprocessing chain to be equalized, compares this transfer function with areference transfer function derived from the reference version of thetest signal and deduces from the differences noted tailoring valuesmaking it possible to attenuate these differences for an equalizationfilter inserted following the processing chain to be equalized.

Advantageously, an equalization filter tailoring circuit operates in anautoadaptive manner, so as to maximize the value of correlation betweenthe two versions of the test signal, the reference version and thatwhich has passed through the processing chain to be equalized, inclusiveof the equalization filter.

Advantageously, the test signal generator provides two versions of thetest signal, one in the input frequency band of the processing chains,the other in the output frequency band of the processing chains.

Advantageously, the reference version of the test signal is provided byone of the processing chains, the so-called main processing chain, whichis devoid of any adjustable equalization filter and of any equalizationfilter tailoring circuit, and whose effective transfer function is takenas reference transfer function, whereas all the other processing chains,the so-called auxiliary processing chains, are provided individuallywith an adjustable equalization filter and with an equalization filtertailoring circuit.

Advantageously, all the processing chains are provided individually withan adjustable equalization filter and with an equalization filtertailoring circuit, the reference version of the test signal being takenequal to a weighted linear combination of the versions of the testsignal that are transmitted by the various processing chains and aretapped off upstream of the adjustable equalization filters.

Advantageously, the tailoring of an equalization filter is done in thecourse of calibration periods during which the useful signal is replacedat the input of the processing chain considered by the test signal.

Advantageously, time slots are reserved for the calibration periodswhich are simultaneous for the various processing chains, the degarblingspatial filtering being interrupted during these time slots.

Advantageously, the calibration periods assigned to the variousprocessing chains are not simultaneous but follow one another insuccession, the processing chain under test being provisionallydiscarded from the spatial filtering.

Advantageously, the test signal is a spread band signal which occupiesthe same frequency band as the useful radio signal and which is obtainedby modulation of a carrier with a specific pseudorandom binary string.

Advantageously, the test signal is a pure frequency line scanning thefrequency band of the useful radio signal.

Advantageously, in the case of parallel processing chains correspondingto reception pathways originating from a receive array antenna withseveral radiating elements, the test signal is injected into the variousprocessing chains, at the output of the receive array antenna, by meansof multiplexing devices making it possible to link the inputs of theprocessing chains either to the receive antenna, or to the test signalgenerator.

Advantageously, the tailoring of the equalization filters of theprocessing chains is done continuously, in the background of thereception of the useful signal, by means of the test signal which isadded to the useful signal at the input of the processing chains by wayof a bank of couplers.

Advantageously, the tailoring of the equalization filters of theprocessing chains is done continuously, in the background of thereception of the useful signal, by means of the test signal which ismodulated by an auxiliary modulation with the aid of a pseudorandombinary string, which is added to the useful signal at the input of theprocessing chains by means of a bank of couplers and which is separatedfrom the useful signal by demodulation at the output of the processingchains.

The subject of the invention is also a device for antigarbling, inreception, of a useful radio signal operating by spatial filtering andimplementing the aforesaid process.

Other characteristics and advantages of the invention will becomeapparent from the description, hereinbelow, of an embodiment given byway of example. This description will be given in conjunction with thedrawing illustrating, diagrammatically, various examples of devices forantigarbling by spatial filtering mounted following a multielementreception antenna having several reception pathways and furnished, inaccordance with the invention, with adjustable equalization filtersinterposed at the output of the processing chains of the receptionpathways, upstream of the antigarbling spatial filter, and with circuitsfor tailoring the equalization filters operating with the aid of a testsignal produced locally.

BRIEF DESCRIPTION OF THE DRAWINGS

More precisely, the drawing comprises:

a FIG. 1 showing an example in which the circuits for tailoring theequalization filters operate, in the course of calibration periods, byspectral analyses of a test signal version tapped off at the output ofthe measurement chain, upstream of the adjustable equalization filter,and of a test signal reference version provided directly by the localgenerator producing the test signal applied to the input of theprocessing chains,

a FIG. 2 showing an example in which the circuits for tailoring theequalization filters operate, in the course of calibration periods,adaptively, on the basis of a correlation between a test signal versiontapped off at the output of a measurement chain, downstream of theequalization filter, and of a test signal reference version provideddirectly by the local generator producing the test signal applied to theinput of the measurement chain,

a FIG. 3 showing an example in which the circuits for tailoring theequalization filters operate, in the course of calibration periods, byspectral analyses of a test signal version tapped off at the output ofthe measurement chain, upstream of the adjustable equalization filter,and of a test signal reference version tapped off at the output of ameasurement chain devoid of any adjustable equalization filter,

a FIG. 4 showing an example in which the circuits for tailoring theequalization filters operate, in the course of calibration periods,adaptively, on the basis of a correlation between a test signal versiontapped off at the output of the measurement chain, downstream of theequalization filter, and of a test signal reference version tapped offat the output of a measurement chain devoid of any adjustableequalization filter,

a FIG. 5 showing an example in which the circuits for tailoring theequalization filters operate, in the background of the reception of theuseful signal, by means of the test signal which is added to the usefulsignal at the input of the processing chains by way of a bank ofcouplers adaptively, on the basis of a correlation between a test signalversion tapped off at the output of the measurement chain, downstream ofthe equalization filter, and of a test signal reference version provideddirectly by the local generator producing the test signal applied at theinput of the measurement chain,

a FIG. 6 showing an example in which the circuits for tailoring theequalization filters operate, in the background of the reception of theuseful signal, by means of the test signal which, after having undergonean auxiliary modulation, is added to the useful signal at the input ofthe processing chains by means of a bank of couplers, by spectralanalyses of a test signal version tapped off at the output of themeasurement chain, upstream of the adjustable equalization filter, andseparated from the useful signal by demodulation, and of a test signalreference version provided directly by the local generator producing thetest signal applied at the input of the processing chains,

a FIG. 7 showing an example in which the circuits for tailoring theequalization filters operate, in the background of the reception of theuseful signal, by means of the test signal which, after having undergonean auxiliary modulation, is added to the useful signal at the input ofthe processing chains by means of a bank of couplers, by spectralanalyses of a test signal version tapped off at the output of themeasurement chain, upstream of the adjustable equalization filter, andseparated from the useful signal by demodulation, and of a test signalreference version tapped off at the output of a measurement chain devoidof any adjustable equalization filter,

a FIG. 8 showing an example in which the circuits for tailoring theequalization filters operate, in the background of the reception of theuseful signal, by means of the test signal which, after having undergonean auxiliary modulation, is added to the useful signal at the input ofthe processing chains by way of a bank of couplers, adaptively, on thebasis of a correlation between two versions of the test signal that areseparated from the useful signal by demodulation and are tapped off, oneat the output of a measurement chain to be equalized, downstream of theequalization filter, and the other at the output of a measurement chaindevoid of any adjustable equalization filter, and

a FIG. 9 showing an example in which the circuits for tailoring theequalization filters operate, in the background of the reception of theuseful signal, by means of the test signal which, after having undergonean auxiliary modulation, is added to the useful signal directly at theinput of each sensor by means of the radiation of the central elementand by directional coupling with the latter, adaptively, on the basis ofa correlation between two versions of the test signal that are separatedfrom the useful signal by demodulation and are tapped off, one at theoutput of a measurement chain to be equalized, downstream of theequalization filter, and the other at the output of a measurement chaindevoid of any adjustable equalization filter.

DETAILED DESCRIPTION OF THE DRAWINGS

In the various figures, the unchanged elements retain the sameindexations.

The device for antigarbling by spatial filtering illustrated in FIG. 1is assumed to process in reception, radio signals originating from thenavigation satellites of the GPS system. It is placed at the level ofthe input stages of a GPS receiver, following the receive antenna 1, infront of the stages devoted to the despreading of the GPS signals, tothe extraction of the information contained in the despread signals andto the calculations of position and of speed on the basis of theextracted information.

The receive antenna 1 is an array antenna with upward directedhemispherical radiation pattern so as to make it possible to pick upsignals originating from transmitters stowed onboard nongeostationarysatellites. It is configured so as to have a skyward sensitivity limitedto angles of elevation greater than a few degrees so as to be influencedas little as possible by intentional or unintentional terrestrialjammers. In the manner in which it is represented, it is formed of anarray of seven radiating elements 1 a, 1 b, 1 c, . . . , 1 f, 1 garranged on a substantially horizontal earth plane, six 1 a, . . . , 1 fdistributed at the vertices of a regular hexagon and a seventh 1 gplaced at the centre of the hexagon. This representation is theoreticalsince the receive array antenna is in fact embodied on the basis of thepatch antenna technique. Other antenna configurations are possible withmore or fewer radiating elements arranged at the vertices of a regularpolygon and with a central radiating element or otherwise, the antennapreferably having a radiation pattern that is omnidirectionalazimuthally since the orientation of the receiver is not necessarilyknown in advance.

The signals picked up by the radiating elements 1 a, . . . , 1 g of thereceive array antenna are applied to identical individual processingchains, only one of which 2 d is represented in full, catering for theshaping of the signal picked up rendering it suitable for purely digitalprocessing. Customarily, each individual processing chain comprises oneor more input stages 20 effecting a limitation of the noise out of bandand a drop in frequency of the signal picked up, from the HF range inwhich it was transmitted (L bands) to an intermediate frequency range FIthat is more suitable for digitization, an analogue/digital converter 21and an output stage 22 completing the frequency drop and ensuring thepassage of the useful signal to baseband.

After having been digitized and placed in baseband by the individualprocessing chains 2 a, . . . , 2 g, the signals picked up by the variouselements 1 a, . . . , 1 g of the antenna 1, which have the form ofstrings of digital samples with two components, one I the so-calledin-phase component and the other Q the so-called quadrature component,are applied to an antigarbling spatial filter 3.

The antigarbling spatial filter 3 carries out a linear combination ofthe signals picked up by the various radiating elements of the antennaso as to create a reception pathway favouring the useful signals to thedetriment of the jamming signals. Its principle of operation relies onthe assumption that the useful signals and the jamming signals are notpicked up in the same directions. It consists in creating, in theradiation pattern corresponding to the reception pathway formed at theoutput of the spatial filter, holes in the directions of the jammers butnot in the directions of the useful signals.

The tailoring of the antigarbling spatial filter 3, that is to say thedetermination of the weighting coefficients of the linear combination ofthe signals picked up by the receive antenna which it carries out, isdone by considering one of the reception pathways applied at the inputof the spatial filter 3 as being a master pathway and the others asbeing auxiliary pathways, by prescribing a unit weighting coefficientfor the master pathway and by determining the weighting coefficients ofthe auxiliary pathways so as to obtain a signal uncorrelated with themat the output of the spatial filter.

In the examples represented, it is assumed that the master receptionpathway is that originating from the central element 1 g of the receiveantenna 1 but this master pathway could originate from some otherradiating element or even result from a particular combination of thesignals picked up by the various radiating elements of the antenna,particular in the sense that it is favourable to the reception of theuseful signals. The auxiliary reception pathways originate here fromeach of the peripheral radiating elements 1 a, . . . , 1 f of thereceive antenna 1 but they could also result from particularcombinations of the signals picked up by the various radiating elementsof the antenna, particular in the sense that they would be rather morefavourable to the reception of jammers. The only limitation is that themaster and auxiliary reception pathways be independent, that is to saythat none of them should reduce to a simple linear combination of theothers.

This antigarbling technique will not be detailed further since it iswell known, in the field of radar by the name of antijamming bycancellation of side lobes or CSLC antijamming. One of the problems thatit poses is that it uses methods for determining the coefficients of theantigarbling spatial filter which, within the framework of broadbanduseful signals such as are spread band signals, are very sensitive tothe disparities of the transfer functions of the various individualprocessing chains providing the signals of the main and auxiliaryreception pathways. This leads to the imposition, during the design ofthe independent processing chains, of severe constraints for equalizingthe performance between all the chains to be complied with in theduration, thereby significantly raising the cost.

To remedy the problem posed by the disparities of the transfer functionsof the individual processing chains and thus succeed either in improvingthe performance of the antigarbling, or in obtaining one and the samelevel of antigarbling with individual processing chains of lower cost, abank of automatic equalization circuits that is coupled to a test signalgenerator 5 and to a gang 6 of switches making it possible, on command,to replace, at the input of the various individual processing chains 2a, . . . , 2 g, the signals picked up by the radiating elements 1 a, . .. , 1 g of the receive antenna 1, with a test signal occupying the samehigh-frequency band, available on an HF output of the test signalgenerator 5, is interposed at the output of the individual processingchains 2 a, . . . , 2 f, in front of the inputs of the antigarblingspatial filter 3.

The bank of automatic equalization circuits incorporates an automaticequalization circuit per reception pathway. Only the one 4 d dedicatedto the reception pathway corresponding to the signal picked up by theradiating element 1 d of the receive antenna 1 is visible in FIG. 1.Each automatic equalization circuit 4 d comprises an adjustableequalization filter 40 d interposed at the output of the processingchain 2 d of the reception pathway to which it is dedicated, upstream ofthe antigarbling spatial filter 3, a circuit 41 d for determining thecoefficients of the adjustable equalization filter 40 d, two spectrumanalyser circuits 42 t and 43 d, one 42 t connected at input, to anoutput FI, at intermediate frequency, of the test signal generator 5,the other 43 d connected at the output of the analogue/digital converter21 d of the pathway 1 d assigned to the automatic equalization circuitconsidered 4 d, and a subtractor 44 d connected at the output of the twospectrum analyser circuits 42 t and 43 d providing the difference notedbetween the two spectra to the circuit 41 d for determining thecoefficients of the equalization filter 40 d.

The gang 6 of switches makes it possible to isolate at will theindividual processing chains from the influence of the outside world andhence the jammers so as to feed them with only the test signal.

The test signal generator 5 produces locally, on an HF output in thehigh-frequency transmission band of the useful signal picked up by thereceiver, and on an FI output in the intermediate frequency bandoccupied by the output signal from the analogue/digital converters 21 ato 21 g of the individual processing chains 2 a to 2 g, two versions ofone and the same spread band test signal with the same spectral width asthe useful signal and with powers matched to the input sensitivities ofthe individual processing chains 2 a, . . . , 2 g and of theequalization circuits 4 d. It can implement a carrier dual-phasemodulated by a pseudorandom binary string.

The test signal, once injected by way of the gang 6 of switches, intothe input of the individual processing chains 2 a, . . . , 2 g in placeof the signals received consisting of the useful signals and of jammingsignals picked up by the radiating elements of the receive antenna 1,passes through the various stages of the individual processing chains,which stages are placed upstream of their analogue/digital converters.It thus traverses all their analogue parts and undergoes, on account ofthe individual processing chains 2 a, . . . , 2 g, the same disturbancesas the useful signals picked up by the receive antenna.

The spectrum analyser circuits 42 t, 43 d deliver the complex frequencyspectra of the signals applied to their inputs. They may operateaccording to all the known techniques of spectral analysis, includingthose using the Fourier transformation in all it variants, and also theso-called “high-resolution” techniques using an approach employing theeigenvalues and eigenvectors of the autocorrelation matrix of the signalor an approach employing autoregressive modelling. In FIG. 1, thespectrum analyser circuits 42 t, 43 d are assumed to carry out an FFT,that is to say a fast Fourier transformation.

One, the spectrum analyser circuit 42 t delivers the complex frequencyspectrum of the version of the test signal which occupies theintermediate output frequency band of the processing chain while issuingdirectly, in digital form, from the output FI of the generator of thetest signal 5 whereas the other 43 d delivers the complex frequencyspectrum of the test signal after it has passed through the processingchain 2 d so it is impaired by disturbances due to this processing chain2 d. The complex frequency spectrum delivered by the spectrum analysercircuit 42 t constitutes a reference transfer function model making itpossible to take account of the imperfections of the test signal withrespect to the flat spectrum sought in the useful band while the complexfrequency spectrum delivered by the spectrum analyser 43 d constitutes ameasure of the transfer function of the processing chain 2 d to withinthe imperfections of the test signal.

The subtractor 44 d subtracts the measured transfer function deliveredby the spectrum analyser 43 d from the transfer function model deliveredby the spectrum analyser circuit 42 t so as to reveal the spectraldisturbances that are to be corrected by the adjustable equalizationfilter 40 d placed at the output of the individual processing chain 2 d.

The circuit 41 d for determining the coefficients of the equalizationfilter performs the conventional calculations for determining thecoefficients of a filter so as to effect the transfer function deliveredby the subtractor 44 d. It may be of any known type and operate, forexample, by inverting the autocorrelation matrix of the spectraldisturbances.

The adjustable equalization filter 40 d is a programmable digital filterbeing situated, as it is, following an individual processing chain 2 d,at a level where the signals are available in digital form. It isadvantageously a finite response filter of FIR type.

For convenience, the spectrum analyser 42 t operating directly on thesignal of the test signal generator 5 has been represented as belongingto each automatic equalization circuit 4 a, 4 b, . . . , 4 g. It goeswithout saying that it may exist as just a one-off and be pooled incommon with all the automatic equalization circuits.

According to a variant, instead of producing a pseudorandom binarystring, the test signal generator 5 may produce a pure frequency line ofcalibrated amplitude which scans the band occupied by the useful signalsat the inputs of the individual processing chains 2 a, . . . , 2 g. Thespectrum analyser circuits 42 t, 43 d then provide complex spectraconsisting of the string of complex samples (amplitude/phase) of thesignals tapped off at the level of the outputs of the analogue/digitalconverters of the individual processing chains for various frequencyvalues scanned by the frequency line of the test signal.

FIG. 2 shows a second exemplary device for antigarbling by spatialfiltering provided with a bank of automatic equalization circuits. Thedifference relative to the antigarbling device described in relation toFIG. 1 lies essentially at the level of the design of the automaticequalization circuits 4 d′ and more particularly of the mode oftailoring the adjustable equalization filter 40 d which is no longerdone on the basis of an explicit measurement of the differencesexhibited by the transfer function of the processing chain with respectto a reference transfer function but according to an adaptive methodthat tends to maximize the value of the correlation existing between thetest signal at the output of the adjustable equalization filter 40 d andits version originating directly from the test signal generator 5, thisversion now being taken on an output BB where the test signal isavailable in digital form, in baseband rather than in intermediatefrequency band, a delay circuit 7 making it possible to correct thedelay ΔT1 taken by the test signal emanating from the processing chainon account of its longer journey and to put the two correlated versionsof the test signal back into phase.

Thus, in place of the spectrum analysers 42 t, 43 d and of thesubtractor 44 d, there is a simple correlator 45 d, one input of whichis connected at the output of the adjustable equalization filter 40 dand the other input of which is connected following the delay circuit 7placed on the output BB of the test signal generator 5. The circuit fordetermining the coefficients of the adjustable equalization filter 41 d′operates by successive approximations, implementing the conventionaltechniques based on the gradient algorithm that are used, for example,in the field of modems to carry out matched filtering of thetransmission links.

FIG. 3 shows a third exemplary device for antigarbling by spatialfiltering provided with a bank of automatic equalization circuits. Thedifference with respect to the antigarbling device described in relationto FIG. 1 lies at the level of the test signal version used for thetransfer function model. This version, instead of originating directlyfrom the test signal generator 5 is extracted from the individualprocessing chain 2 g assigned to the reception pathway originating fromthe central radiating element 1 g of the antenna 1 taken as masterpathway for the antigarbling spatial filter 3. In this case, theindividual processing chain 2 g of the master pathway is taken asreference and comprises no automatic equalization circuit unlike all theother individual processing chains 2 a to 2 f. It is pointed out thatthe two versions of the test signal that are used by an automaticequalization circuit 4 d are extracted at one and the same level of thetwo individual processing chains, either, as represented, at theintermediate output of the analogue/digital conversion stage, or even atthe final output, after the stage 22 g, 22 d for translating the signalto baseband, but upstream of the adjustable equalization filter 40 d.

FIG. 4 shows a fourth exemplary device for antigarbling by spatialfiltering provided with a bank of automatic equalization circuits. It isdistinguished from the previous one by the construction of the automaticequalization circuits 4 d′ which borrows from that with correlator 45 dof the second example illustrated in FIG. 2. As one of the inputs of thecorrelator 45 d must be connected at the output of the adjustableequalization filter 40 d delivering a baseband signal, the other inputof the correlator 45 d can now be connected only at the level of thefinal output of the individual processing chain 2 g although the signalis also in baseband.

In the examples of devices for antigarbling by spatial filteringprovided with a bank of automatic equalization circuits which have justbeen described in relation to FIGS. 1 to 4, the banks of automaticequalization circuits are adjusted in the course of calibrationoperations where the test signal is substituted with the signals of thereceive antenna 1 by throwing a bank of switches 6. These calibrationoperations may be done simultaneously for all the processing chains, forexample when the receiver is turned on or sequentially after a certainperiod of time taking account of the drifting of the processing chains.By assigning different calibration periods for the processing chains itis possible to let the spatial filtering continue during the calibrationperiods, only the processing chain under test being provisionallydiscarded.

The insertion of calibration periods in the course of the operation ofthe GPS receiver is advantageously entrusted to an automaton whicheither regularly prescribes calibration periods when the receiver isturned on and after periods of operation of a specified duration chosenas a function of the rates of drifting of the characteristics of theindividual processing chains, or initializes a calibration period whenthe receiver is turned on and whenever the signal-to-noise ratio of thesignal received, measured downstream in the receiver after thedespreading operation becomes less than a threshold regarded as thetolerable minimum for reliable operation of the GPS receiver.

It is also possible for antigarbling devices with spatial filtering tobe provided with a bank of automatic equalization circuits that adjustthemselves continuously, in the background of the reception of theuseful signal.

FIG. 5 gives an example thereof derived from that of FIG. 2. This fifthexample is much like that of FIG. 2 from which it borrows the design ofthe automatic equalization circuits 4 d′. It is distinguished therefromonly by the fact that the bank 6 of switches 6 a to 6 g has given way toa bank 8 of couplers 8 a to 8 g. The test signal produced at highfrequency by the test generator 5 is superimposed on the receptionsignals of the antenna 1 in the various individual processing chains 2 ato 2 g. At the level of an automatic equalization circuit 4 d′, theuseful reception signal superimposed on the test signal at the output ofthe adjustable equalization filter 40 d plays the role of noise in thecorrelation performed by the correlator 45 d, with the version of thetest signal originating directly from the test signal generator 5, whichnoise is eliminated or, at the very least, greatly weakened if one hastaken the precaution of taking for the test signal a pseudorandom binarycode orthogonal to the pseudorandom binary codes used by the GPS system.

FIG. 6 shows a sixth exemplary device for antigarbling with spatialfiltering together with a bank of automatic equalization circuits thatcontinually adjust themselves, in the background of the reception of theuseful signal. This other example is much like that of FIG. 1 from whichit borrows the design of the automatic equalization circuits 4 d. It isdistinguished therefrom nevertheless, by various characteristics. As inthe previous example of FIG. 5, the bank 6 of switches 4 a to 4 g hasgiven way to a bank 8 of couplers 8 a to 8 g but additionally, the testsignal available on the HF output of the test signal generator 5undergoes auxiliary modulation by a pseudorandom code before beingsuperimposed on the useful reception signal at the input of theindividual processing chains 2 a to 2 g followed by auxiliarydemodulation at the output of the individual processing chains 2 a to 2g before being applied to the spectrum analyser 43 d.

The auxiliary modulation and demodulation of the test signal before andafter its passage through the individual processing chain to beequalized are aimed at making it possible to attenuate the useful signalat the input of the spectrum analyser 43 d so that the latter seesmainly the test signal. They are carried out by means of a localpseudorandom binary code generator 9, of a modulator 10 interposedbetween the HF output of the test signal generator 5 and the bank 8 ofcouplers 8 a to 8 g and of a demodulator 11 interposed in front of theinput of the spectrum analyser 43 d of the automatic equalizationcircuit. The auxiliary modulation is preferably done at a multiple ofthe bit rate of the test signal and carries out an additional bandspreading which is chosen so as to adapt to the band width margin oneither side of that of the useful signal, adopted for the individualprocessing chains during their design.

FIG. 7 shows a seventh exemplary device for antigarbling by spatialfiltering provided with a bank of automatic equalization circuits. Thedifference relative to the antigarbling device described in relation toFIG. 6 above lies at the level of the test signal version used for thetransfer function model. This version, instead of originating directlyfrom the test signal generator 5 is extracted from the individualprocessing chain 2 g assigned to the reception pathway originating fromthe central radiating element 1 g of the antenna 1 taken as masterpathway for the antigarbling spatial filter 3.

Like the signal extracted from the individual processing chain 2 dforming the subject of the equalization, the signal extracted from theindividual processing chain 2 g taken as reference undergoesdemodulation at 12 intended to pick out the test signal from the wholeset of components of the output signal from this individual processingchain 2 g. It is pointed out that, as previously with regard to theexample illustrated in FIG. 3, the individual processing chain 2 g ofthe master pathway taken as reference comprises no automaticequalization circuit unlike all the other individual processing chains 2a to 2 f and that the two versions of the test signal that are used byan automatic equalization circuit 4 d are extracted at one and the samelevel of the two individual processing chains, either, as represented,at the intermediate output of the analogue/digital conversion stage, oreven at the final output, after the stage 22 g, 22 d for translating thesignal to baseband, but upstream of the adjustable equalization filter40 d.

FIG. 8 shows an eighth exemplary device for antigarbling by spatialfiltering provided with a bank of automatic equalization circuits. It isdistinguished from the previous one illustrated in FIG. 7 by theconstruction of the automatic equalization circuits 4 d′ which borrowsfrom that with correlator 45 d of the examples illustrated in FIGS. 2, 4and 5. As one of the inputs of the correlator 45 d must be connected atthe output of the adjustable equalization filter 40 d delivering abaseband signal, the other input of the correlator 45 d can now beconnected only at the level of the final output of the individualprocessing chain 2 g although the signal is also in baseband.

Other variants of devices for antigarbling by spatial filtering with abank of automatic equalization circuits are also possible, in particularvariants combining the bank of switches and the bank of couplers of theexamples described above. It is also possible to take into account thepart of the inhomogeneity defects, in the processings of the receptionpathways, that are due to the radiating elements of the receive antenna.To do this, it is sufficient to replace the banks of switches or ofcouplers by a transmit antenna excited by the local test signalgenerator and arranged on the same carrier as the receive antenna, inproximity to or even within the receive antenna.

As shown by the ninth exemplary device for antigarbling by spatialfiltering illustrated in FIG. 9, one of the radiating elements of thereceive antenna, here the element 1 g, may be used as transmit antennafor the test signal. This radiating element 1 g which retains its roleon reception, is excited on transmission, by means of an auxiliarycoupler 8″ by the test signal previously subjected to auxiliarymodulation in order to allow its separation from the useful signalduring its reception.

It is pointed out that, in the various devices described, the banks 4,4′ of automatic equalization circuits intervene on the signals althoughthey have been digitized. They are therefore implemented by digitalprocessing like the antigarbling spatial filter 3 so that the twofunctions of equalization by temporal filtering and of antigarbling byspatial filtering may be conducted by means of one and the same circuitspecialized in signal processing such as a DSP (Digital SignalProcessor).

The devices just described have the benefit of allowing the correctionof defects in the frequency-dropping analogue chains for the receptionpathways of an array antenna by programming digital hardware componentswhich is a much easier operation than intervening in the design of theanalogue stages. It is thus possible to circumvent the often veryformidable specifications imposed on the HF stages in the field ofarrays of sensors of radio signals such as radionavigation signals.

The corrections performed digitally rely on easily modifiablecalculation algorithms that are obviously less subject to technologicallimitations than those encountered in HF.

Finally, from the cost point of view, this tradeoff in complexity fromanalogue to digital makes it possible, for identical performance, tosubstantially reduce the development costs of the products and leavesthe way open to the possibilities of optimization by simple updating ofsoftware even in low-cost applications.

1. A method for processing a radio signal, comprising: receiving asignal by a set of processing chains; equalizing an output of at leastone processing chain using an adjustable temporal equalization filter,based upon differences between a test signal processed by the processingchain and a reference version of the test signal; and spatial filteringthe equalized radio signal processed by each processing chain.
 2. Themethod according to claim 1, wherein the test signal processed by the atleast one processing chain has appreciably greater power than thereceived radio signal.
 3. The method according to claim 1, wherein thetest signal processed by the at least one processing chain isdistinguished from the received signal by an auxiliary modulation. 4.The method according to claim 3, further comprising modulating the testsignal by means of a pseudorandom binary string.
 5. The method accordingto claim 1, further comprising: measuring a transfer function of theprocessing chain to be equalized using the test signal; comparing themeasured transfer function with a reference transfer function derivedfrom the reference version of the test signal; calculating coefficientvalues of the adjustable equalization filter based upon differencesbetween the measured transfer function and the reference transferfunction; and tailoring the equalization filter with the coefficientvalues to attenuate the measured differences.
 6. The method according toclaim 5, wherein calculating the coefficients further comprises:maximizing a value of correlation between the test signal and thereference version of the test signal in an autoadaptive manner.
 7. Themethod according to claim 1, further comprising generating the testsignal and the reference version of the test signal, the test signal inan input frequency band of the processing chains, and the referenceversion of the test signal in an output frequency band of the processingchains to be equalized.
 8. The method according to claim 1, furthercomprising transmitting the reference version of the test signal througha main processing chain which is devoid of any adjustable temporalequalization filter or any equalization filter tailoring circuit.
 9. Themethod according to claim 1, further comprising filtering the signalreceived from each processing chain by the adjustable temporalequalization filter and using an equalization filter tailoring circuit.10. The method according to claim 9, wherein the reference version ofthe test signal is equal to a weighted linear combination of the testsignal transmitted by the processing chains and tapped off upstream ofthe adjustable temporal equalization filter.
 11. The method according toclaim 1, further comprising tailoring the adjustable temporalequalization filter using an equalization filter tailoring circuitduring calibration periods when the received signal is replaced, at theinput of the processing chain concerned, by the test signal.
 12. Themethod according to claim 11, further comprising reserving time slotsfor the calibration periods, the calibration being performedsimultaneously for all processing chains, interrupting the spatialfiltering during these time slots.
 13. The method according to claim 11,further comprising assigning the calibration periods in sequence to eachof the processing chains and provisionally disregarding the processingchain under test from spatial filtering.
 14. The method according toclaim 1, further comprising: generating the test signal which is aspread band signal and occupies the same frequency band as the receivedsignal; and generating the test signal by modulation of a carrier with apseudorandom binary string.
 15. The method according to claim 1, furthercomprising generating a pure frequency test signal by scanning thefrequency band of the received signal.
 16. The method according to claim1, further comprising: switching inputs of the processing chains toeither a receive antenna or to at least one local test signal generator;and receiving the test signal at the inputs of the processing chains.17. The method according to claim 1, further comprising adding the testsignal to signals received from radiating elements of an antenna arrayby means of coupling devices and receiving the added test signal at theinputs of the processing chains.
 18. The method according to claim 1,further comprising transmitting the test signal to processing chains bya transmit antenna mounted on a same carrier as the receive antenna. 19.A device for processing a radio signal, comprising: a set of parallelprocessing chains having identical transfer functions, each parallelprocessing chain having an input and an output; a spatial filterperforming a weighted linear combination of the output of the parallelprocessing chains; a test signal generator for generating a test signal;at least one circuit for injecting the test signal provided by the testsignal generator into each input of the parallel processing chains; anda bank of automatic equalization circuits, each equalization circuitarranged at the output of each parallel processing chain.
 20. The deviceaccording to claim 19, wherein each automatic equalization circuitcomprises: a first spectral analysis circuit configured to analyze aspectrum of the test signal processed by each processing chain; a secondspectral analysis circuit configured to analyze a spectrum of areference version of the test signal; a subtractor circuit having anoutput based upon subtracting the spectrum measured by the firstspectral analysis circuit from spectrum measured by the second spectralanalysis circuit; an adjustable temporal equalization filter interposedbetween the output of the processing chain and the spatial filter; and acircuit for determining coefficients of the adjustable temporalequalization filter based upon the output of the subtractor circuit. 21.The device according to claim 20, wherein the test signal generatorgenerates a first version of the test signal in a frequency band of theinput to the processing chains and generates the reference version ofthe test signal in a frequency band of the output of the processingchains.
 22. The device according to claim 20, further comprising a mainprocessing chain, which is one of the parallel processing chains and thedoes not comprise any automatic equalization circuit, the mainprocessing chain delivers the reference version of the test signal. 23.The device according to claim 20, further comprising a bank of automaticequalization circuits, each automatic equalization circuit correspondsto one processing chain included the set of parallel processing chains.24. The device according to claim 23, wherein the reference version ofthe test signal is based upon a weighted linear combination of the testsignals received from the outputs of the processing chains.
 25. Thedevice according to claim 20, wherein the automatic equalization circuitfurther comprises: an adjustable temporal equalization filter having anoutput interposed between the output of each processing chain; acorrelator configured to compare the reference version of the testsignal with the test signal received from the output of the adjustabletemporal equalization filter and outputting a correlation value; and anequalization filter tailoring circuit configured to maximize thecorrelation value.
 26. The device according to claim 25, wherein thetest signal generator generates a first version of the test signal in afrequency band of the inputs to the processing chains and a referenceversion of the test signal in a frequency band of the output of theadjustable temporal equalization filters.
 27. The device according toclaim 25, further comprising a main processing chain, which is one ofthe parallel processing chains and does not comprise any automaticequalization circuit, the main processing chain delivers the referenceversion of the test signal.
 28. The device according to claim 25,further comprising a bank of automatic equalization circuits, eachautomatic equalization circuit corresponds to one processing chainincluded in the set of parallel processing chains.
 29. The deviceaccording to claim 28, wherein the reference version of the test signalis based upon a weighted linear combination of the test signals receivedfrom the outputs of the processing chains.
 30. The device according toclaim 19, comprising a bank of switches, which are configured to switchthe input of the processing chains from a signal received from radiatingelements to the test signal, during calibration periods.
 31. The deviceaccording to one of claim 30, further comprising an automaton configuredto introduce the calibration periods.
 32. The device according to claim30, wherein the automaton is configured to introduce the calibrationperiods after a specified duration of operation.
 33. The deviceaccording to claim 30, wherein the automaton is configured to introducethe calibration period each time signal-to-noise ratio downstream of thespatial filter goes below a predetermined threshold value.
 34. Thedevice according to claim 19, comprising a bank of couplers, which areconfigured to add the test signal to signals received from the radiatingelements.
 35. The device according to claim 19, wherein the parallelprocessing chains are configured to connect to radiating elements of areceive antenna, which includes a transmit antenna configured totransmit the test signal.
 36. The device according to claim 35, furthercomprising a directional coupler configured to connect the transmitantenna with a processing chain.
 37. The device according to one ofclaim 35, further comprising an automaton configured to introduce acalibration period.
 38. The device according to claim 35, wherein theautomaton is configured to introduce a calibration period after aspecified duration of operation.
 39. The device according to claim 35,wherein the automaton is configured to introduce a calibration periodeach time signal-to-noise ratio downstream of the spatial filter goesbelow a predetermined threshold value.
 40. The device according to claim19, wherein the test signal generator is a pseudorandom binary stringgenerator.
 41. The device according to claim 19, wherein the test signalgenerator is configured to generate signal with predetermined frequencyband.
 42. The device according to claim 19, wherein the device isconfigured to be integrated with a satellite navigation system receiver.